In certain vital signaling and control systems, such as, cab signal speed control systems for railway and mass and/or rapid transit operation, various vehicle-carried apparatus and circuits are employed for being selectively responsive to one of a plurality of speed command input signals. The input signals are picked up from the wayside and each is coded to indicate the authorized speed at which the train or vehicle should be moving in any given section of track along its route of travel. The picked-up signals, which are made up of a carrier frequency and a selected one of a number of code rates or frequencies, after being amplified and demodulated, are applied to a code following relay termed the master relay. This relay, in turn, drives the code filters which are selectively responsive to the particular code rate. These filters drive decoding relays to provide a given cab signal indication to the trainman or operator in the cab of the locomotive or head vehicle. It will be appreciated that in such cab signal speed control systems, it is mandatory for the trainmen or operator to take appropriate action within a given period of time after a more restrictive speed command is received onboard from the wayside. Thus, upon reception of a more restrictive or lower speed command signal, the operator must immediately acknowledge a warning signal and begin decelerating the moving train to the newly received prescribed speed or the emergency brakes will be automatically set after an elapsed period of time to bring the train to a complete stop. It will be appreciated that the acknowledgement of the warning signal and the deceleration of the moving train or transit vehicle should take place as soon as possible after reception of a more restrictive signal in order to prevent the train or vehicle from going too fast and too far beyond a safe braking and stopping point. Further, in order to provide the highest degree of safety to individuals as well as to afford the most protection against damage to the equipment, it is essential to ensure that under no circumstance will a critical component or circuit failure be capable of producing an unsafe condition. In the past, the acknowledgement was performed by contact logic of separate vital electromagnetic relays which were activated by the pressing of the button of an acknowledgement switch after the receipt of a downward speed command which results in the deenergization of the brake relay and causes the cessation of the alarm. It will be appreciated that in order to accomplish these functions, it was previously necessary to employ three vital relays for a three-speed control system, four vital relays for a four-speed control system and so on. While these previous relay type of acknowledging circuits operated in a safe and satisfactory manner, they were extremely heavy, unduely bulky and prohibitively expensive. In order to alleviate the shortcomings of the relay circuits, it is highly advantageous to develop a solid-state electronic acknowledging circuit which is light in weight, small in size and low in cost. It will be appreciated that in addition to performing all the functions of previous electromagnetic circuits, an electronic acknowledgement circuit must also operate in a fail-safe manner. That is, any vital solid-state circuit must meet the criteria of the Association of America Railroads (A.A.R.) which define fail-safe, a term used to designate a railway signal design principle, the object of which is to eliminate the hazardous effects of a failure of a component or system. Thus, a vital piece of apparatus, circuit or system is considered to operate in a fail-safe manner when any conceivable failure of any critical component or part results in a condition at least as restrictive or as safe as that preceding the failure.